Wave guide phase shifter



June 10, 1952 5, FQX 2,599,753

WAVE GUIDE PHASE SHIFTER Filed Jan. ll, 1946 POLARIZATION POLAkIZATlO/VPOLARIZATION VECTOR 1 VECTOR 1 VECTOR E? F I63 FIG. 3A 2 DIELECTRIC sueFIGS

INVENTOR 2L By A 6. FOX I 1 -2x 7 ATTORNEY Patented June 10, 1952 WAVEGUIDE PHASE SHIFTER Arthur Gardner Fox, Eatontown, N. .L, assignor toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application January 11, 1946, Serial No. 640,498

1 Claim. 1

This invention relates to the transmission of electro-magnetic waves andmore particularly to phase shifters for use in connection with waveguides.

An object of the invention is to simplify and improve the constructionand operation of phase shifters including, especially, adjustable phaseshifters.

A feature of the invention is a section of wave guide comprising aconductive boundary, an enclosed dielectric medium and longitudinallydisposed therein a plate of dielectric material having a dielectricconstant substantially different from that of said medium.

A feature of the invention is a microwave phase shifter comprising asection of wave guide loaded by a plate of high dielectric constantdisposed longitudinally in the guide section. The wave transmissioncharacteristics will differ for mutually perpendicular orientations oflinearly polarized waves referred to the dielectric plate as a referenceaxis.v

Another. feature of the invention is a wave guide phase shifter having adielectric plate therein adapted to provide a predetermined phase shiftover a broad band of frequencies.

A further feature of the invention is a wave guide phase shifter thatalters the phase velocity of the waves having a certain polarization bya substantially greater amount than waves polarized perpendicularthereto.

Still another feature of the invention is a reflectionless wave guidephase shifter having a low loss dielectric plate therein proportioned totransform a linearly polarized wave into a circularly polarized wave.

Other objects, features and advantages will be apparent from thedetailed specification taken in connection with the accompanyingdrawings wherein:

Fig. 1 shows a cylindrical wave guide;

Fig. 1A shows a wave guide phase shifter having an internal dielectricplate in accordance .with the invention;

Figs. 2, 2A, 2B show explanatory electric force diagrams;

Figs. 3, 3A show a modification of the wave guide phase shifter;

Figs. 4, 4A show modifications of the dielectric plate element; and

Figs. 5, 5A show a modified wave guide phase shifter for square pipes.

Phase shifters involving a section of wave guide provided with internaldiametrical rods suitably spaced apart, have been disclosed in theUnited States patent applications, Serial No.

464,333, filed November 3, 1942, by A. G. Fox, now 1 Patent No.2,438,119, issued March 23, 1948, and

Serial No. 469,897, filed December 23, 1942, by D,

H. Ring, now Patent No. 2,425,345, issued August,

12, 1947. The phase shifting action in these devices is developedessentially from the transmission properties of a single frequency orrelatively narrow band wave filter.

Phase shifters involving a section of wave guide waves polarizedperpendicular thereto, have been disclosed in the United States patentapplication of W. A. Tyrrell, Serial No. 590,365, filed April 26, 1945,now Patent No. 2,546,840, issued March 27, 1951. q

The phase shifters in accordance with the present invention overcome thelimitations inherent in critical resonance or narrow band phaseshifters, and are characterized by the broad-band transmissioncharacteristics of a uniformly loaded wave guide such'as is disclosed inthe United States Patent 2,199,083 issued April 30, 1940, to S. A.Schelkunoff.

In accordance with an embodiment of the in vention a phase shiftercomprises a section of wave guide having a dielectric plate or septum Itherein, such as polystyrene or the like, with a high dielectricconstant'for providing a prede-- termined shift in the phase of a wavepropagated therethrough. The dielectric plate will alter the phasevelocity of all polarizations of waves. However, waves linearlypolarized parallel thereto will be retarded to a greater degree thanwaves polarized perpendicular thereto. In practice it is desirable to sodesign the plate that the velocities of the waves of these two principlepolarizations are as unequal as possible, so that the phase shiftdifferential between them will be as great- The plate may be dimensionedand shaped to provide a predetermined difi'erential' 'de-* as possible.

phase shift, and more particularly a'90 or gree differential shift.

The'phase shifter may be rendered practically reilectionless bycapacitive reactance screws, so

oriented and adjusted as to provide a compem sating and neutralizingreflection.

The term dominant wave as used in this specification denotes a wavecorresponding to that particular mode having the lowest possible 1dicated arrow; 1

the guide. At the same instant, the. phase at 1A and 3A, results in analtered cut oif, wave- 3 cut-off frequency in a pipe of predeterminedcross-section.

The term linear polarization as applied to wave guides, denotes a stateof the electromagnetic field, wherein the electric force vector at 5 thecenter of the wave guide cross-section, executes as a function of time,simple harmonic motion on astraight line. 7

Similarly, the terms circular and elliptical polarization arecharacterized by an electric force 10 V 1 separate phase velocities,corresponding to waves seamed parallel and perpendicular to the plane ofthe plate, respectively, as illustrated in vector at the center of theguide executing a cir cular or elliptical sweep respectively asa-function i of time variation. U

Analytically, an elliptical polarized wave may be compounded from twolinearly polarized waves i of the same frequency, whose axes ofpolarization,

are perpendicular and whose relative amplitudes and phases aredifferent. Accordingly, fronrthisviewpoint circular polarizationrepresents a 2.1291711 and 2,129,712 both issued September 13,

1938, to which reference isherewith made.

.Thephase velocity or propagation of waves in anrempty-hollow pipe waveguide is given by V r c a X r V 1 Where 0 equals the volocity of lightin vacuo, A equals the wavelength in vacuc and )\c equals the critical,or cut-off wavelength associated with the propagation of a particularwave mode and a particular cross-section of pipe. H 4 *From the formulait is apparent that the phase velocityo in a hollow pipe wave guide willalways bef i'eatr than c and that 'u approaches c as the-wavelength 7 ismade small relative to M thecut-off wavelength. r Referring toFig. l,-which shows a cylindrical wave guide pipe? *of uniform circularcrosssection, assumethat input waves-of the dominant mode'are appliedthereto at the left and propagate therethroughin the directionof the in-5o Atanygiven instant, the wavesat any arbitrary cross-section A willhave a certain phase PA, with respect to some fixed reference point insome other cross-sectionB will correspondingly be ins. a a The phasedifference between points A and B will thenbeexpressed-by the formulasB-a=1- 360= 3a 2 9 where; V Ag=the wavelength within the guide;

f=thefrequency of the wave oscillations; and

L=distance between A, B..

From Equation 2, it is apparentthat 'for a given frequency; the phasedifference between two given points is dependent only upon the phasevelocity .v. 7 V 7 The introduction of. a. longitudinal dielectric plateI into the circular guide,as shown in Figs.

length As and phase velocity v', respectively,

waste-as the dielectric plate definitely alters 0 formly the phasevelocity and critical cut-off frequency for polarizations ororientations of electric field parallel thereto, it has practically amuch smaller effect on the corresponding perpendicular polarizations.For such perpendicular polarizations the plate may be considered toproduce a second altered cut-off wavelength M" and phase velocity v". J

Therefore, a guide section with a longitudinal dielectric plate may beconsidered to have two Figs. 2A and 213, respectively.

Ihe phase difference between points A and B for waves polarized parallelto the plate is whereas for the perpendicular polarization e 1 A"={# seo4) The differential phase shift, that'is, the difference in phasebetween transmitted waves in the two orientations is given by ,The termdifierential phase -s'hifti-a's used herein, denotes a difference in theelectrical length of, two transmission paths. In the wave guide phaseshifter disclosed, these two' paths are present within the same vsectionof, wave guide and result from the introduction of the dielectric platetherein, which may be considered the analog of a transmission linewithuni distributed capacitive loading. The distributed capacitive loadingaifects differently the transmission of the two sets o flinea'rlypolarized waves, whose axes of polarization are mutually perpendicular.Having chosen a particular dielectric for the material of theplate and aparticular shape therefor, both of which factors determine a specificdifference infphase velocities, then any'desiredphaseshiftmay beachieved by a suitablechoice of the longitudinal dimension or length L.

Such a diiferential phase'shift section may the present inventioncomprises a low loss'dielectric plate I of polystyrene, attached withinfthe guide section so as to possess both 'raldial'and longitudinalextent as illustrated in Fig. 'lA, The plate 1 may extend only part wayacross the guide section like a fin or completely in a diametr'al plateas shown, and more preferably should possess ahigh dielectric constant,a f i Thereby a pair of geometrical *axesjmutually perpendicular is 'setup within "the wave guide" or 0 wave guide section, to whichwillfco'rrespon'dthe aforementioned different propagationandt'ra'nsmission characteristics, lTheeff'ect ofthe diametral plate Ion wave transmission will accord.-

mg1yjde enu'upc itsori'entatioh with'respectfto "the polarization of thewaves. r'or "simplicity hereinafter, the discussionwill be limited tothe dominant modeof Wave propagation.

' Fig. 2 shows the approximate configuration of the electric lines offorce in, a uniform circular wave guide as set up by a linear polarizedwave. Fig. 2A shows qualitatively the electric force configurationestablished by thepresence of a diametral dielectric plate in the waveguide, wherein the plane of the plate coincides with the direction ofpolarization as indicated; while Fig. 2B represents the lines of force,when the plate is perpendicular to the direction of polarization. Theseindicate that the field pattern and hence the transmissioncharacteristics are more profoundly altered when the plate is parallelto the electric field.

In some practical applications, the phase shifter may form an integralpart of a main wave guide ormay be inserted therein or connected theretoas a rotatable joint in the manner more fully disclosed in the UnitedStates application of A. G. Fox, Serial No. 464,333, filed November 3,1942, now Patent No. 2,438,119, issued March 23, 1948.

The manner of operation of these wave guide 1 phase shifters may beinferred from the general discussion above, and will now be describedwith more particularity.

The projection of the plate into the wave guide section along adiametral plane thereof acts to increase the cut-ofi wave-length for theparallel polarization case shown in Fig. 2A, and according to Equation1, decreases the phase velocity relative to the values appropriate forFigs. 2 or 2B.

Let it be assumed that a linearly polarized, dominant wave is incidentupon a phase shifter and let the angle between the axis of polarizationand the plane of the dielectric plate be denoted by p. The incident wavemay be regarded as the resultant of two linearly polarized components inphase, whose axes of polarization are parallel and perpendicular,respectively, to the plane of the plate. The relative amplitudes of theorthogonal components will be related as cosine 13 to sine B. Thecomponents will propagate through the wave guide phase shifter withdiffering phase velocities, the component parallel to the dielectricplate traveling slower. In their progress therethrough the componentsacquire a phase difference, which increases with the dis tance of wavepenetration into the phase shifter. The total phase shift developedbetween the two components will depend essentially upon the length ofthe plate, its thickness and the dielectric constant of the material ofwhich it is made and by proper proportioning of these dimensions andfactors, any desired phase shift may be secured. When thecomponentsemerge from the phase shifter, therefore, the resultant willin general have been transformed from the input linearly polarized waveto an output elliptically polarized wave.

There are two special cases, however, which are of special importance,namely, the 90-degree phase shifter and the 180-degree phase shifter.Thus, when a linearly polarized wave is incident upon a 90-degree(differential) phase shifter having the plane of the dielectric plate atan angle ,8 of 45 degrees to the axis of polarization, the emerging wavewill come out circularly polarized. For other values of the angle ,9 thewaves will, in general, emerge elliptically polarized.

The 180-degree phase shifter possesses the significant property that anincident, linearly polarized wave will emerge linearly polarized for alla: values of the angle 5. However, in general, a change in orientationof the polarization vector will result. 7 Y s As previously disclosed, alongitudinal dielectric plate causes not only a change in phase velocitybut also in characteristic impedance relative to the-uniform wave guidesection. An abrupt transition in impedance properties between theprincipal guide and the phase shift section would result in. undesirableand disturbing reflections.

To eliminate or to neutralize, and minimize such effects, capacitivereactance screws 2 (Fig. 3), as disclosed in United States Patent No.2,432,093 issued December 9, 1947, to A. G. Fox, are provided to set upcounteracting reflections, whereby the overall transmission will beessen-. tially reflectionless. I

Alternatively, the phase shifting dielectric'plate may be madeinherently reflectionless by means of impedance matching terminalportions, disclosed in Figs. 4 and 4A. In Fig. 4, the dielectric. plateI has tapered terminal portions 3 adaptedv to provide a smoothtransition and match into the impedance of the principal guide.Alternatively, the impedance matching terminals may be in the form ofatapering notch 4 as shown in Fig. 4A.

It has been stated earlier that in general the presence of a dielectricplate affects the phase velocities of waves polarized both parallel withand perpendicular to the plate. If, however, the plate is very thin,then the waves polarized perpendicular to the plate are affected only toa negligible extent. In order for such a plate to be effective, it musthave a very high dielectric constant. For dielectrics of moderatedielectric constant such as are in common use, a plate of negligiblethickness would, have negligible effect on waves of both polarizations.Consequently plates of appreciable thickness must generally be used. Onthe other hand, if the plate is made so thick that it completely fillsthe waveguide, then waves of both polarizations will be slowed downequally and there will be no difierential phase shift. Consequentlythere must be a plate of some intermediate thickness which will producea maximum of differential phase shift.

This optimum thickness of plate can, of course, be determined eithertheoretically or experimentally. However, the theory of wave guidespartially filled with solid dielectric is so complex that it ispreferable to determine the optimum plate thickness experimentally. Thismay readily be done by starting with a thick plate and shaving it downin steps, measuring the differential phase shift at each step, and inthis way determining a curve of differential phase shift versus platethicknes from which the optimum plate thickness may be read off. Forpolystyrene plates in circular or square wave guide the optimumthickness will be of the order of but somewhat less than half thediameter or width of the wave guide.

Whereas the dielectric material referred to'in this specification hasbeen primarily polystyrene, it should be understood that other low lossdielectrics preferably of high dielectric constant may be used in lieuthereof such as polystyrene loaded with lead chloride, rutile, metallictitanates, and ceramics of strontium titanate or calcium titanate.

Dielectric plates of increasing thickness may be utilized to shorten thelength L of the phase shift section required to provide a predeterminedphase-sh m. "Various stapes :for the conductiveUnited'States-application "O'f Al GwFox sl ihl NO.

461,333, filed November 3, 1942.

While the invention has been-iilustratedin specific forms for thepurpose 'of'the disclosure; it will b'e apparent that modificationsthereof or therein may bem'ade by the persons skilled "in the art'v'rithout departing from the puvpcse and s'oe'peof the invention.

What is claimed is:

:A llrefiectiomessphase shifter for incident linearly polarized,dominant electromagnetic wavesmomprisinga; non-radiating, lowlosssection of uniform hoilow wave guide, a low loss dielectric platedisposed longitudinally between the end's of said section, collinearreactance screws disposed in the wall of said guide trans-- y saiaipietqsamipiete iiein asposedobn uei with respectt'o'"iaheineidentpolarization and having a thickness small with respect toileng'thiiiiid width, whereby poiarization components 'ipamlie'l andperpendicular to said plate respectively are

